Metal plating of plastics



United States Patent 01 zfice 3,524,754 METAL PLATING F PLASTICS GeorgeC. Blytas, Kensington, Edward S. Slott, Berkeley, and Edward R. Bell,Lafayette, Califi, assignors to Shell Oil Company, New York, N.Y., acorporation of Delaware No Drawing. Filed Apr. 28, 1967, Ser. No.634,483 Int. Cl. C23c 3/02 US. Cl. 117-47 14 Claims ABSTRACT OF THEDISCLOSURE Organic polymer substrates are conditioned for electrolessdeposition of metal coatings by impregnating their surface layer to adepth of at least about microns with a metal such as copper or nickel,diffused into the surface layer from solution of a salt of said metal inan organic solvent which has a dissolving or swelling action on theplastic, followed by reduction in situ by means of contact with areducing solution.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to deposition of metal on organic polymers. More particularly,it relates to a novel economical method for sensitizing thermoplastics,including nonpolar crystalline plastics such as polyolefins, toelectroless metal deposition from chemical plating solutions. Theinvention also relates to the novel articles resulting from said method.

Description of the prior art Electroless metal deposition, also calledelectroless or chemical metal plating, refers to chemical deposition ofan adherent metal coating on a non-conductive, semiconductive orconductive substrate in the absence of an external electric source.

It is desirable for many commercial purposes to apply metal coatings tothermoplastic surfaces, e.g., in order to provide electricallyconductive surfaces or surface paths, or to provide decorative orprotective coatings.

The recent state of the art is detailed in a serially published surveyElectroplated Plastics-a Comprehensive Survey of the Current Position,by W. Goldie, in Electroplating and Metal Finishing, vol. 18 (1965),414417, 428 (December), and vol. 19 (1966), 3-7 (January), 49-53(February), 97-100 (March), 133-137 (April), 185-188 (May) et seq.

Although many kinds of plastics have been metalplated on an experimentalbasis, only special plating grades of ABS polymers have enjoyedsubstantial commercial success to date as plating substrates, apparentlydue to the fact that the present of polar nitrile groups facilitatesproduction of metal coatings having the desired degree of adhesion.

Known published and proprietary commercial methods for electrolessplating of non-conductive or semi-conductive surfaces comprise a largenumber of separate steps, as many as 20 to 30, generally including mostor all of the following: various washes to clean the substrate surface;chemical or physical treatments to provide a controlled amount ofsurface irregularity or roughness or a chemical modification of thesurface layer of polymer; surface treatment by immersion in an aqueoussensitizing bath such as acidified stannous chloride; seeding orcatalyzing by immersion in an activating bath from which there aredeposited on the thermoplastic surface catalytic nucleating centers of ametal which catalyzes the deposition of the desired metal coating--theactivating bath is generally an aqueous acidified solution of a noblemetal halide,

3,524,754 Patented Aug. 18, 1970 e.g., of gold, platinum or palladium,which is reduced to metal by stannous ions adsorbed on the substrate orby reducing agents contained in the subsequent electroless metaldeposition bath; and thereafter electroless deposition of a continuous,conducting coating of a metal such as copper, nickel or cobalt byimmersing the activated substrate in an electroless plating bathcontaining a salt of the metal to be plated and a suitable reducingagent in aqueous solution. Articles plated in this manner can then beelectroplated, if desired, by known electroplating methods, with a widevariety of metals. In commercial practice, a number of furthermanipulations intervene, such as controlled rinses between the treatingsteps.

The conventional methods summarized above have numerous disadvantageswhich are well known to persons skilled in the art. These methods areexpensive to use, due to the need for a large number of manipulativesteps and for a number of separate treating baths which may requirefrequent replacement. Achievement of reproducible results is difiicultin such complex processes. Plating of nonpolar thermoplastics by theprior art methods is particularly difficult.

SUMMARY OF THE INVENTION It is the principal object of this invention toprovide a simplified method for conditioning thermoplastic substrates,including non-polar ones such as polyolefins, for electroless depositionof metals from conventional electroless plating solutions. A furtherobject is to provide a simple, effective, economical method formetal-plating plastic articles. Other objects will become apparent fromthe following description of the invention.

The objects of this invention are achieved by the method of impregnatingthe surface layer of a clean thermoplastic substrate to a depth of atleast about 5 microns with a metal such as copper or nickel, diffusedinto the surface layer from solution of a salt of said metal in asolvent which has a dissolving or swelling action on the thermoplasticpolymer. The diffusion step is followed by contacting the substrate witha reducing solution capable of reducing cations of metal in thesubstrate surface layer to zero valence. Substrates which containdiffused metal in accordance with this invention are capable of beingmetal-plated by contact with electroless chemical plating solutions ofthe prior art, followed, if desired, by conventional electroplating ofthe chemically metal-plated substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical embodiment of theinvention is illustrated by the following schematic flow diagram:

(1) Impregnation Immerse a clean thermoplastic substrate in a solutionof a salt of copper or nickel in a solvent which has dissolving orswelling action on the polymer. Control conditions to cause diffusion ofmetal salt into the polymer surface layer only.

(2) Drying Dry the treated substrate.

(3) Reduction Reduce at least part of the metal ions in the substrate tozero valence by immersing the dried substrate in a reducing solution atreducing conditions.

(4) Washing Remove remaining reducing solution by suitable wash, e,g.,water.

() Electroless plating Apply a continuous coating of copper or nickel byimmersing the substrate in an electroless plating solution.

The method of this invention is applicable to polymer substrates in anyshape. For example, it can be employed to impregnate and coat finelydivided particles of polymer, fibers or films of polymer, moldedarticles and extruded shapes. The method is especially adapted forplating articles of substantial thicknesse.g., 50 mils and greater. Themetal impregnation affects only a few mils of the outer surface, thusleaving the bulk of the article unaffected, permitting it to retain thephysical and chemical properties of the polymer, e.g., its strength andstability.

A substantial advantage of the invention is that it permits metalcoating of run-of-the-mill polymers rather than requiring the use ofspecial polymer formulations.

The process of this invention is applicable to an organic polymers whichare capable of being swelled or dissolved by a solvent in which asuitable metal salt is soluble. Polymers for use in this inventioninclude non-conducting thermoplastic addition polymers or condensationpolymers, and elastomeric polymers.

This invention is of particular advantage in the plating of polymerswhich contain no polar groups, because these are particularly difiicultto plate by known methods. The method of this invention is thusparticularly advantageous in the plating of polyolefins such aspolyethylene, polypropylene, and stereoregular polymers of higherolefins. Other suitable polymers comprise polyvinyl aromatics such aspolystyrene and its copolymers; ABS (copolymer of acrylonitrile,butadiene and styrene); polymers and copolymers of acrylonitrile, vinylchloride, vinyl acetate, and the like; acrylic polymers such aspolymethyl methacrylate; and condensation polymers such as polyesters,polyamides, and polyester amides. Suitable elastomers comprisecopolymers of butadiene and styrene (SBR), polybutadiene, rubberyethylene-propylene copolymers (EPR) and the like, provided they arecapable of being swollen by a solvent.

One of the outstanding features of this invention is that it does awaywith the multiple sensitizing and activation solutions employed in theprior art processes for preparing non-conductive substrates forelectroless plating. Instead, this process uses a relatively simpleabsorption of a metal salt, preferably of copper, into the surface layerof the substrate, followed by reduction to convert at least part of themetal ions to zero valence.

Optional pretreatment steps In general, the best plating results areobtained when there are no significant irregular internal strains in thesubstrates. Substrates containing internal strains clue to the method ofpreparation are preferably first annealed. For example, in the case ofpolypropylene articles of /8 inch thickness containing some irregularstrains, annealing in boiling water for 24 hours, prior to any contactwith solvent, resulted in coating of improved evenness. Annealing ispreferably carried out at a temperature within about F. of the highesttemperature employed in the subsequent processing steps.

The substrates, prior to impregnation in accordance with this invention,should be reasonably clean. No elaborate special cleaning procedures arerequired.

In the case of substrates which are relatively resistant to solventpenetration, such as isotactic polypropylene, it may be of advantage topretreat the substrate with an active solvent prior to the impregnationstep. This appears to make its surface layer more receptive to the metalsalt, as Well as accomplising a simple cleansing function. Such pretreatis suitably carried out at about the same temperature and with the samesolvent employed in the impregnation step, by a dip lasting a fewseconds. Such pretreatment may also be accomplished by exposure tosolvent vapor.

Other treatments which make the substrate more receptive to impregnationor improve adhesion of the metal coating may be employed. For example,pretreating polypropylene with 50% aqueous sulfuric acid appeared toresult in some improvement in the peel strength of the final product, asillustrated in Example 6.

The impregnation step and optional pretreating steps It is essentialthat the impregnation bath have some solvent or swelling action on thesubstrate, since it is only from such a solution that the metal saltwill penetrate into the surface layer.

In the impregnation step, treating conditions and solution componentsare selected to provide penetration of metal salt into the surface layerof the substrate to a depth of at least 5 and no more than about 200m.Preferably the depth of penertation is about '5-80/L. Deeper metalpenetration into the substrate would provide no advantages in theprocess of this invention and may be of disadvantage because the moreintensive solvent action required to achieve such penetration may damagethe surface of the substrate, and the larger amount of metal in thepolymer may affect its physical or chemical properties in an undesirablemanner.

It will be readily apparent that different solvents and differenttreating conditions will be required for different polymers. Forexample, impregnation of polystyrene can be readily accomplished byimmersion for about 10 seconds or less at room temperature in a bathcontaining 202'5% w. of a suitable copper salt (cuproustrifiuoroacetate) in benzene. impregnation of polypropylene requiresmore severe conditions, e.g., immersion for about 30 seconds at l00l10C. in a bath containing 815% w. of the same copper salt in xylene.

For best results, polypropylene impregnation should be carried out froman aromatic solvent bath at a tempera ture of at least about C.,suitably at from 110- 1l5 C.

In general, it is desirable to select the impregnation bath andconditions such that impregnation is completed in less than about 1%minutes, preferably in less than 1 minute and more preferably in fromabout 5 to about 30 seconds. Suitable impregnating baths generallycontain impregnating metal ions such as Cu+ or Cu++ in solution inconcentrations from about 2% wt. to the upper solubility limit, or toabout 20% wt. Relatively high concentrations of metal salte.g.,saturated solutions-are preferred for the impregnation of easilysolvent-attacked polymers such as polystyrene. Such high saltconcentrations permit quick introduction of the desired amount of metalinto the polymer surface layer and also desirably reduce thesolvent-activity of the liquid. For less readily solvent-attackedpolymers, such as polypropylene, it has been found desirable to employlower metal salt concentrations in the impregnation step, e.g., from15-16% metal, unless the polymer has been given a solvent-pretreat asdescribed below, in which case relatively high metal salt concentrationsare also suitable.

The impregnation step is desirably controlled to produce at least about0.02 gram of metal per square foot of treated surface and preferablyabout 0.08 gram per square foot. Amounts of about 0.25 g./ft. aresatisfactory and the suitable range extends to 0.5 g./ft. More metal maybe introduced but is generally not required.

Components of the impregnation bathsSolvents Effective solvents for usein this process are those which have the required swelling or solventaction on the substrate to be impregnated and are capable of dissolvingthe desired metal compound in a homogeneous, single phase, liquid systemat the desired treating temperature.

Typical solvents for use with polystyrene are aromatics such as benzeneand toluene, or substituted aromatics such as chlorobenzene or benzylalcohol.

Solvents suitable for impregnation baths for polypropylene comprisearomatics and other cyclic hydrocarbons which remain liquid at thetreating temperature of at least about 100 C., e.g., toluene, xylene,tetrahydronaphthalene and decahydronaphthalene; benzene could beemployed under sufficient pressure to keep it liquid at the treatingtemperature.

Mixed solvent systems are sometimes of advantage. For example, in thetreatment of polystyrene it may be desirable to add from 33-50%, andsuitably from -20% of a polar organic compound. Such addition has beenfound to permit greater latitude in the treatment of polystyrene withsalts such as cuprous trifluoroacetate. In general, polar compoundswhich are desirable for use With aromatics in the impregnation ofpolystyrene are compounds which have a dielectric constant in excess of10 and which do not reduce the metal salt in the impregnating bath.Suitable compounds include propylene carbonate, ethylene carbonate andvarious acetals, lactones and nitriles.

For impregnation of ABS polymers from aromatic impregnating baths, itwas found desirable to include an amount of a lower aliphatic nitrile,e.g., acetonitrile or propionitrile, in excess of about 1%, e.g., about2% by weight of the solution, in benzene. Other nitriles and otheraromatics can also be employed in the impregnation of ABS. Even purenitriles can be used for ABS or other polyarylonitrile-containingpolymers.

For impregnation of polymethyl methacrylate a suitable solventcomposition consists of benzene with 12% by weight dichloroethane. Otheraromatics containing minor proportions of other chlorinated hydrocarbonscan be employed. Other solvents suitable for impregnation baths forpolymethyl methacrylate include methyl methacrylate monomer and otherorganic esters.

Components of the impregnation baths-Metal salts The requirements for asuitable metal salt for use in this invention are that the salt must besoluble in the impregnation bath, capable of diffusing into thesubstrate, and capable of being reduced in the substrate to zerovalence.

Preferred for use in the invention are organic cuprous and cupric salts.Also useful are salts of nickel, silver and cobalt.

A group of copper salts which are particularly useful are the fluorooroxy-cuprous salts of the general formula CuXA, wherein the moiety XArepresents an anion in which X is an oxygen or fluorine atom and A isthe remainder of the anion. In general, the anions of these salts areinorganic, organic or organo-inorganic acids,

having equivalent weights of no more than about 1000 and preferably nomore than 200. Representative suitable anions comprisefluoro-substituted carboxylates, e.g., trifluoroacetate; sulfate;benzene sulfonate; ethylsulfonate; fluorosulfonate; nitrate,difluorophosphate and acid phosphates; diamino phosphate,perfluoroborate; hexafluorophosphate; hexafluoroantimonate, andchloroalanate; these cuprous salts tend to disproportionate under theinfluence of moisture; the salts and their solutions must be handled inthe absence of contact with the atmosphere or with water.

Other suitable copper salts are cuprous salts of carboxylic acids, e.g.,cuprous formate, acetate, lactate, benzoate, salicylate and di-tert.butyl salicylate. Cuprous chloride has been successfully employed inpropionitrile solution in the impregnation of ABS polymer.

Other suitable copper salts are cupric salts of carboxylic acids, e.g.,the naphthenate, undecylenate, oleate, salicylate, and the di-tert.butyl salicylate.

Among the cuprous and cupric carboxylates, those of acids having anolefinic or aromatic group are preferred.

It will be understood that not all salts will give equal- 1y goodresults, due to difference in properties such as solubilityrelationships, diffusion rates, ease of reduction,

(3 relative ease of being leached out of the substrate and the like.

In the event that the impregnating metal salt is sensitive to componentsof the atmosphere, e.g., oxygen or moisture, it is necessary to carryout the first part of the process, from impregnation through reduction,in the absence of such components, e.g., in an inert gas atmosphere.

If desired, a rinse may be interposed between the impregnation anddrying steps, suitably utilizing the same solvent as is employed in theimpregnation bath.

The drying step The drying step is not critical. Its purpose is toremove solvent which adheres to and which may be sorbed in or on thesubstrate, so that it will not interfere with the following reducingbath. Conditions to accomplish this may differ for different polymersand solvents, but are readily determined. For example, very short dryingperiods, e.g., 1060 seconds at room temperature and atmosphericpressure, are suflicient for polystyrene impregnated with a benzenesolution of cuprous trifluoroacetate, while longer periods, e.g., 4 to15 minutes at room temperature and atmospheric pressure, or a shortexposure to vacuum or to a hot gas stream, are preferred forpolypropylene impregnated with a xylene solution of the same salt.

The reduction step The components of the reducing solution and theconditions in the reduction step are selected to provide controlledreduction of at least a substantial part of the metal salt in thesurface layer of the substrate to zero valence. It is not necessary toachieve complete reduction of all the metal salt in the substrate.

Components of the reducing bath The solvent employed in the reducingbath should be relatively inert to the substrate, since relatively longcontact times are generally required. The reducing agent should besufiiciently strong and present in sufficient concentration to reduce atleast a substantial part of the metal ions in the substrate to zerovalence.

The ease of reducibility of the particular salt impregnated in thesubstrate and the strength of bonding between the salt and substratewill affect the selection of suitable reducing agents.

The reducing bath is usually capable of dissolving the metal salt beingreduced and is therefore capable of leaching metal salt out of thesubstrate. In relatively polar substrates, e.g., polystyrene, there issufiicient bonding between the substrate and the metal salt to preventsubstantial leaching during the reducing step. In non-polar substrates,such as polypropylene, it is important to control conditions duringreduction so as to prevent excessive leaching before the desired amountof reduction has taken place in the substrate. This can be accomplishedby adopting relatively drastic reduction conditions, for example, highertemperature and higher concentration, at which reduction occurs fasterthan leaching. For example, reduction in polystyrene is satisfactoryusing 23% W. hydrazine in isopropyl alcohol at 25 C., while reduction inpolypropylene is preferably carried out at about C. with 48% hydrazinein isopropyl alcohol.

A preferred reducing agent for use in this invention is hydrazine (N HOthers which have been successfully employed to reduce cuprous salts insubstrates are aluminum triisopropyl and benzaldehyde. Other reducingagents can be selected from those kown in the art, e.g., those describedin Electroless Copper Plating by E. B. Saubestre in Proceedings of theAmerican Electroplaters Society, vol. 46, pp. 264276 (1959).

A preferred solvent for the hydrazine reducing bath is anhydrousisopropyl alcohol. Hydrazine may also be employed in propylenecarbonate, Water, or other solvents which do not attack the hydrazine orthe substrate.

7 Washing On completion of the reduction step, the article is removedfrom the reducing bath and rinsed or washed to remove any adhering orsorbed components which might interfere with the following plating bath.In most cases water is a suitable wash or rinse medium. It may bedeionized or distilled; two or more successive washes or rinses may beemployed, as required to remove potential plating contaminants to thedesired extent.

Applying continuous metal coating After impregnation and reduction asdescribed, a thin adherent continuous metal coating can be applied tothe substrate by immersion in any desired electroless chemical platingbath.

For practical reasons, copper is usually the preferred metal applied byelectroless coating; nickel is next preferred. Nickel requires muchhigher temperatures (80 C. or more) for electroless plating. Copper isdeposited at room temperature. Also, copper is solderable which isimportant, for example, in electronic applications. Other metals whichmay be applied in this manner include gold, silver, cobalt, and mixturessuch as nickel-copper and nickel-cobalt.

The chemically metal-coated substrate may then be subjected toelectroplating in the conventional manner, either to apply a thickercoating of copper or nickel or to apply a protective or decorativecoating of metals such as chromium, copper, nickel, tin, cadmium,cobalt, silver, gold, platinum-group metals, etc.

Both electroless plating and electroplating are well known processes.Detailed descriptions of electroless plating and electroplating,including formulations of plating baths and conditions for conductingsuch plating, are found in reference works such as Modern Electroplatingby F. A. Lowenheim, John Wiley and Sons, 1963, 2nd edition, and inreferences cited therein. Electroless plating, including suitableformulations of plating baths for copper, nickel, silver and gold isalso described by Goldie, supra, vol. 19, pp. 6-8.

A very common electroplating succession is to electroplate with levelingcopper, then with nickel, then with chromium. The thicknesses usuallyapplied are in the order Cu Ni Cr.

EXAMPLES The invention is illustrated, but is not to be consideredlimited, by the following examples. Parts and percentages in theexamples and throughout the specification are by weight unless otherwiseindicated. Room temperature is about C. The illustrative experiments arecarried out, unless otherwise stated, on 2 inch by 2 inch coupons cutfrom about inch thick sheet of the polymer being tested.

An indication of the effectiveness of the plating is the peel strengthof the plate, which is determined on samples which have been coated to atotal thickness of about 3 mil with ductile copper. The peel strengthtest employed in these examples consisted of placing the sample in aspecial jig on an Instron tensile testing machine and pulling a 1 inchwide strip of the metal layer at an angle of 90 from the surface,utilizing a cross-head speed of 1 inch per minute. Peel strength isreported in pounds per inch of width (lbs/in.)

Example 1 Coupons of commercial high impact polystyrene containing asmall amount of SBR (conventional styrene-butadiene rubber) are dippedin an impregnating bath consisting of 25% cuprous trifluoroacetate inbenzene at room temperature. The samples are withdrawn from the bath,dried in a still nitrogen atmosphere at room tem perature for secondsand then immersed for 10 minutes in a 3% solution of hydrazine inanhydrous isopropyl alcohol at room temperature. The samples arewithdrawn from the reducing bath, washed three times with distilledwater, and then plated in a conventional electroless bath having thefollowing typical compositions:

G./l. CuSO 3.3 HCHO 8 NaOH 4 Rochelle salt 17 Na CO 1.6 NiCl (optional)1.5

The samples are kept in the electroless plating bath until at least aconductive continuous layer of copper has been plated out. A layer ofless than one mil in thickness is suflicient. The samples may then befurther plated by conventional electroplating with additional copper orother metals.

Samples prepared as above, electroplated with copper to 3 mil totalthickness, have a plating peel strength of about 5 lbs./in.

Example 2 The procedure of Example 1 is repeated except that theimpregnation bath consists of 25% cuprous trifluoroacetate in a mixedsolvent of benzene and 20% propylenecarbonate.

Samples produced utilizing this impregnation bath have an improvedplating peel strength, in the range of 6-8 lbs/in.

Example 3 Substituting coupons of crystal grade polystyrene for the highimpact polystyrene in Examples 1 and 2 results in plated articles havinga plating peel strength of /2 t0 1 lb./in. less in each case.

Example 4 Samples of commercial isotactic polypropylene are immersed ina xylene bath at C. for 30 seconds to one minute. The samples are thenimpregnated and further treated as in Example 4. The plating peelstrength of the resulting products is over 5 lbs./in.

Pretreating the samples in xylene permits successful impregnation withan impregnating bath having a higher concentration of cuproustrifluoroacetate, e.g., 15-20% than that desirable for the procedure ofExample 4. Substituting such a more concentrated impregnating bath forthat of Example 4 results in still further improvement in peel strengthof the plated polypropylene.

Example 6 Samples of commercial isotactic polypropylene are pretreatedby immersion in 50% aqueous sulfuric acid at 80 C. for about one minute.The samples are then washed with water to remove adhering acidcompletely, and are further treated by the procedure of Example 4. Theresulting plated polypropylene has peel strengths in the range of 5-6lbs/in.

Example 7 In the plating of polystyrene according to the generalprocedure of Example 1, the impregnating and reducing baths arereplaced, respectively, with a saturated solution of cupric naphthenateand 5 to 8% W. solution of hydrazine in anhydrous isopropyl alcohol. Thesamples are maintained in the reducing bath for about 20 minutes at roomtemperature or for about 10 minutes at about 45- 50 C. The platedsamples have peel strengths in excess Example 8 Samples of commercialABS polymer, both of a plating grade and of a non-plating grade, areimpregnated by being immersed for to seconds in a solution of 10% w.cuprous chloride in acetonitrile. The samples are dried for 3 to 5minutes in a still nitrogen atmosphere at room temperature and thenimmersed in a reducing solution of 23% hydrazine in isopropyl alcohol.The samples are then Washed three times with distilled water and platedaccording to the procedure of Example 1. The plated samples have platingpeel strengths of over 2 lbs/in. Similar results are obtained withsamples impregnated with 10% cuprous trifluoroacetate in benzenecontaining 2% acetonitrile.

Omission of acetonitrile from the plating bath results in insufiicientimpregnation under otherwise identical conditions.

Example 9 Samples of commercial polymethyl methacrylate are impregnated,being immersed for about 60 second in a bath consisting of cuproustrifluoroacetate in benzene containing 2% ethylene dichloride. Thesamples are withdrawn, dried in a still nitrogen atmosphere for at leastseconds and immersed in a reducing bath of 24% hydrazine for 10-20minutes. The samples are then washed three times with water and platedaccording to the procedure of Example 1. The peel strength of theplating is more than 2 lbs/in. Similar results are obtained with animpregnating solution of 20% cuprous trifluoroacetate in methylmethacrylate.

Example 10 A strip of styrene-butadiene rubber is successfully platedwith flexible copper plating by following the procedure of Example 1,except for substitution of 5-10% aqueous N H solution as the reductionbath.

Example 11 Injection molded 2 /2 inch x 2 /2 inch x /s inch plaques areprepared from isotactic polypropylene and from blends of isotacticpolypropylene with 20% SBR (conventional styrene-butadiene rubbercontaining 23.5% styrene) and with 17% rubbery polyisobutylene,respectively, The samples are annealed 12 to 24 hours in boiling water.The samples are then dried, dipped about 10 seconds in reagent gradexylene at 110 C., removed and dipped 5 sec- Onds in an impregnation bathconsisting of 30% w. solution of cupric undecylenate in xylene, at 110C. The samples are withdrawn from the impregnation bath, allowed to dryin air at room temperature, and then immersed 30 minutes in a reducingbath consisting of 6% hydrazine in anhydrous isopropyl alcohol. They arewithdrawn from the reducing bath and washed several minutes in runningwater at to C. to remove reducing agent and any loose copper particles,and then placed in a chemical copper plating bath such as that ofExample 1. Conductive coatings are deposited in less than 2 hours ofimmersion. The samples are then electroplated with ductile copper to atotal thickness of 3 mils and the peel strength determined. Peelstrengths are in the range from 5 to 6 lbs./in.; samples of theSBR-containing compositions generally have somewhat higher peelstrengths than samples of 100% polypropylene, whilepolyisobutylene-containing compositions show no improvement in peelstrength over straight polypropylene.

We claim:

1. A method of producing a metal-coated article of organic polymericcomposition, comprising the steps of (a) impregnating a metal-freesubstrate of said polymeric composition to a depth of 5 to 200 micronswith a reducible copper salt by contact of said substrate with asolution of 2 to 20% by Weight of said salt in an organic solventcapable of exerting a swelling or dissolving action on said substrate;

(b) reducing at least a substantial proportion of metal cations in theresulting impregnated substrate to zero valence; and

(c) coating the resulting metal-containing substrate with a metal bycontact with an electroless chemical plating bath.

2. A method of plating an article of organic polymeric composition, atleast 50 mil in thickness, which comprises:

(a) contacting the surface of said article which is to be plated andwhich is metal-free with an impregnating solution comprising 2 to 20% byweight of reducible copper salt dissolved in an organic solvent capableof exerting a swelling or dissolving action on said polymericcomposition, until said salt has penetrated the surface of said articleto a depth of from 5 to 200 microns;

(b) removing the article from contact with said impregnating solution;

(0) contacting the impregnated article with a reducing solution adaptedto reduce the metal ions of said salt in said polymeric compositionuntil at least a substantial part of said metal ions has been reduced tozero valence;

(d) removing the article from contact with said reducing solution andwashing off adhering solution;

(e) and thereafter immersing the washed article in an electrolessplating solution, whereby a metal coating is deposited on said surface.

3. The method according to claim 2 wherein said impregnating solutionconsists essentially of an aromatic hydrocarbon solvent and an organiccuprous or cupric salt capable of dissolving in said solvent and ofdiffusing into said polymeric composition.

4. The method according to claim 3 wherein said metal salt is selectedfrom the group consisting of fiuorocuprous and oxycuprous salts havingthe formula CuXA, wherein X is an oxygen or fluorine atom and A is ananion of an organic acid having an equivalent weight no more than 1000,and cuprous and cupric salts of carboxylic acids having an olefinic oraromatic group.

5. The method according to claim claim 2 wherein said polymericcomposition is predominantly polystyrene;

said impregnating solution comprises essentially a reducible organiccopper salt and an aromatic hydrocarbon solvent;

said contact with impregnating solution takes place at about roomtemperature and lasts no more than 10 seconds;

said reducing solution comprises essentially hydrazine and an inertsolvent; and

said contact with reducing solution takes place at about roomtemperature.

6. The method according to claim 5 wherein said impregnating solutionconsists of a reducible organic copper salt, an aromatic hydrocarbonsolvent, and from 3 t 50% of a polar organic compound having adielectric constant greater than 10 which does not reduce the coppersalt in the impregnating bath.

7. The method according to claim 5 wherein said impregnating solutionconsists of a reducible organic copper salt in benzene containing from10 to 20% by weight of ethylene carbonate or propylene carbonate.

8. The method according to claim 5 wherein said impregnating solutionconsists of from about 20% to 25% by weight of cuprous trifluoroacetatein benzene, and said reducing solution consists of about 2% t about 3%by weight of hydrazine in isopropyl alcohol.

9. The method according to claim 2 wherein said polymeric composition ispredominantly polypropylene;

said impregnating solution comprises essentially a reducible organiccopper salt and an aromatic hydrocarbon solvent;

said contact with impregnating solution takes place at a temperature ofat least about 100 C. and lasts from to 100 seconds;

said reducing solution comprises essentially hydrazine in an inertsolvent; and

said contact with reducing solution takes place at a temperature 'of atleast about 60 C.

10. The method according to claim 9 wherein said impregnating solutionconsists of from about 8% to about 15% cuprous trifluoroacetate inxylene and said reducing solution consists of about 4% to about 8% byweight hydrazine in isopropyl alcohol.

11. The method according to claim 9 wherein, immediately prior to saidcontact with impregnating solution, said surface which is t be plated iscontacted at a temperature of at least about 100 C. with an aromatichydrocarbon solvent.

12. The method according to claim 11 wherein said contact with saidimpregnating solution takes place at a temperature of at least 110 C.

13. The method according to claim 2 wherein said polymeric compositionis predominantly a copolymer of acrylonitrile, butadiene and styrene,and said impregnating solution comprises essentially a reducible organiccopper salt in a solvent comprising from 1 to by weight of a loweraliphatic nitrile admixed with from 99 to 0% of aromatic hydrocarbons.

14. The method according to claim 2 wherein said polymeric compositionis predominantly polymethyl methacrylate and said impregnating solutioncomprises essentially a reducible organic copper salt in a solventcomprising a major proportion of aromatic hydrocarbon and a minorproportion of a chlorinated hydrocarbon.

References Cited UNITED STATES PATENTS 2,355,933 8/1944 Weiss 117352,511,472 6/1950 Kmecik 117--160 2,690,402 9/ 1954 Crehan 117-160 X3,093,509 6/1963 Wein 11747 X 3,259,559 7/1966 Schneble et a1 117212 X3,377,174 4/1968 Torigai l17-47 X 3,414,427 12/1968 Levy 11747 3,425,9462/ 1969 Emons 11747 X 3,437,507 4/1969 Jensen 117-47 3,442,683 5/1969Lenoble et a1 117160 X FOREIGN PATENTS 539,621 9/1941 Great Britain.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

mg I J NITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.5, 5 75 Dated August 97 Inventor(s) George C. Blytas-Edward R.Bell-Robert S. Slott It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the heading, the name of the patentee SLOTT is corrected to read"ROBERT S. SLOTT".

5 fir-D SIf'I-E H 3; a

SIGNED AND SEALED NOV. 3, 1970 (SEAL) Attest: Eawaummh AumingO mm 1:.:12.

Willow 01 Patonte

